1. Field of the Invention
The present invention relates to a method for forming a metal wiring of a semiconductor device, and more particularly to a method for forming a metal wiring of a semiconductor device capable of forming a metal plug buried in deeper contact holes having different depths simultaneously with the formation of the metal wiring.
2. Description of the Prior Art
As a semiconductor device has an increased integration degree, its contact hole has a decreased diameter and an increased depth. The contact hole serves to bring an upper conduction layer into contact with a lower conduction layer. Where the upper conduction layer is made of aluminum, there is a limitation on depositing the upper conduction layer on the contact hole having a critical dimension by use of an aluminum sputtering process. In other words, a void is formed in the aluminum layer because the deposition of aluminum on the contact hole is carried out at a low rate in a region near the lower portion of the contact hole and at a high rate in a region near the upper portion of the contact hole. Such a void results in an increase in contact resistance. Moreover, a problem exists in that the aluminum layer may be easily eroded at its portion near the void by the lapse of time.
In order to solve the above-mentioned problems, there has been proposed the formation of a metal plug in the contact hole. This conventional technique will be described in detail, in conjunction with FIGS. 1 to 3.
In accordance with this method, a field oxide film 2 is formed on a semiconductor substrate 1, as shown in FIG. 1. A transistor (not shown) is also formed at an active region of the semiconductor substrate 1. Over the entire exposed surface of the resulting structure, a first insulating film 3 is then formed. Thereafter, a lower conduction wiring 4 is formed on a portion of the first insulating film 3 disposed over the field oxide film 2. Over the entire exposed surface of the resulting structure, a second insulating film 5 and a third insulating film 6 are formed in a sequential manner. Formation of contact holes 10 and 11 is then carried out in order to bring an upper metal wiring to be subsequently formed into contact with the silicon substrate 1 and the lower conduction wiring 4. It can be found that the depth of contact hole may be varied depending on the contact position of the upper conduction wiring.
Where a semiconductor device has deep contact holes having different depths such as the contact holes 10 and 11 of FIG. 1, a void may be formed in a metal layer buried in deeper contact holes such as the contact hole 10 of FIG. 1. In accordance with the illustrated method, formation of a metal plug 22 in the deeper contact hole 10 is carried out in order to prevent the generation of the void, as shown in FIG. 2. The formation of the metal plug 22 is achieved by growing a selective metal film, for example, a tungsten film to a predetermined thickness in the deeper contact hole 10 by use of a chemical vapor deposition (CVD) process. In order to prevent the tungsten film from being grown in the contact hole 11, a photoresist film 20 is coated on the contact hole 11 prior to the formation of the metal plug 22.
Thereafter, the photoresist film 20 is removed, as shown in FIG. 3. Under this condition, an aluminum layer is deposited to a predetermined thickness over the entire exposed surface of the resulting structure, thereby forming a metal layer 23 for forming the upper conduction wiring. The metal layer 23 is buried in the contact holes and coated on the third insulating film 6.
As the metal layer 23 is formed under the condition that the metal plug 22 has been formed, no void is formed in portions of the metal layer buried in the contact holes.
Subsequently, the metal layer 23 is subjected to a photoresist film patterning process and an etching process so that it is patterned.
Although the above-mentioned conventional method prevents the formation of a void, it has a problem of additional procedural steps because the photoresist film should be coated on the contact hole having a smaller depth prior to the formation of the metal plug and subsequently be removed. Furthermore, the method requires the use of a masking process upon forming the metal wiring after the deposition of the metal layer. In addition, where a subsequent treatment after the formation of the metal plug is delayed, an oxide film may be formed on the surface of the metal plug. This oxide film results in an increase in electrical resistance of the metal film subsequently formed.